1. Field of the Invention
This invention relates to a surface treating method wherein various types of materials can be surface-treated with a stream of corona discharge to impart hydrophilic properties or functional groups to the material or to subject the material to a kind of coating or etching.
2. Description of the Prior Art
Several methods for surface modification or surface treatment are known in the art wherein the surface of substrate materials is treated so as to impart thereto hydrophilic properties, water repellence and adhesiveness. For instance, the surface treatment of vulcanized rubber has been adopted for making composite materials by bonding the resultant vulcanized rubber with rubber materials or other materials such as metals, resins and the like or for pretreatments for coating of the vulcanized rubber. For this purpose, there are known many surface treating techniques.
Among the surface treating methods of vulcanized rubber, there is known a method of imparting adhesiveness to vulcanized rubber wherein the surface of vulcanized rubbers is strongly oxidized with strong acids or strong oxidizing agents, thereby causing the entire surface to be finely cracked. However, this method has the problem that care should be fully taken in handling the strong acids or oxidizing agents and that the vulcanized rubber surface is considerably impeded by the action of the strong acid or oxidizing agent, with the bonding force being not always satisfactory.
Moreover, there have been proposed surface treating methods including a method in which vulcanized rubber is exposed to chlorine gas or is immersed in water through which chlorine gas has been passed, or a method using pseudo-halogen compounds (Japanese Patent Publication No. 52-36910). In all of these methods, the rubber is attacked at double bonds thereof to incorporate Cl atoms instead, thereby forming a modified surface which is susceptible to bonding. Where vulcanized rubbers are surface treated according to these methods, the treated surface is converted into a kind of resin, for example, at the time of fabricating a rubber vibration insulator by using the rubber such as NR/SBR rubber in combination with other materials such as metals or resins. Thus, there arises the problem that the adhesiveness and heat resistance become deteriorated, coupled with another problem that the treated surface undesirably suffers yellowing. Moreover, if the above methods are applied to fabrication of golf balls wherein a vulcanized rubber made primarily of a balata material (or transpolyisoprene) is used to make a golf ball cover and the ball cover is subjected to the surface treatment and coated, the resultant ball becomes poor in appearance. The use of chlorine gas or pseudo-halogen compounds will cause an ecological problem.
Another surface treating method includes the one wherein a gas such as O.sub.2 or a mixed gas such as CF.sub.4 and O.sub.2 is used for activating the surface of vulcanized rubber by etching according to a low pressure glow plasma treatment. Although the low pressure glow plasma treatment ensures a uniform surface treatment, with a reduced degree of unevenness of the treatment, it is necessary to effect the low pressure plasma treatment ordinarily at a pressure as low as not higher than 10 Torr. When the treatment is carried out on the industrial scale, a large vacuum apparatus is essential. During the course of evacuation to reduced pressure, oils and water will be released from the vulcanized rubber surface, with the possibility that intended performance and function are not obtained by the plasma treatment. In addition, the plasma treatment has another problem that heat is liable to generate during the treatment, so that application of the treatment to materials made of low melting substances is difficult.
Like the above low pressure glow plasma methods, corona discharge methods which have been put into practice are not satisfactory in their treating effect.
To solve the problems of the prior art methods or techniques, we have already proposed in Japanese Laid-open Patent Application No. 3-23183 a method wherein the vulcanized rubber surface is subjected to atmospheric pressure plasma treatment using a gas which consists of molecules Containing a halogen atom or an oxygen atom. In the case, the surface treatment can be performed simply in a clean environment since any solvent is not used. The resultant vulcanized rubber has an adhesive surface which is better than those surfaces obtained by prior art methods using, for example, a low pressure glow plasma treatment. In addition, since the treatment is limited only to the surface layer, the physical properties of the vulcanized rubber per se are not impeded.
In the atmospheric pressure plasma method, however, it is preferred to dilute a gas for treating a rubber with a diluent gas in order to stabilize the discharge. To this end, it will be necessary to use expensive gases such as helium gas in large quantities. The impedance at the time of the discharge is lower than that of corona discharge but is higher than in the case of low pressure plasma. This makes it difficult to use general-purpose corona power supply or low pressure plasma power supply as it is. A specific type of power supply for the atmospheric pressure plasma is required in most cases, leading to the problem of the rise in treating costs.